As is known, integrated circuits are fabricated on wafers via multi-layer fabrication processes. This involves a series of processing steps such as cleaning, impurity doping, material deposition, and selective etching. A number of these steps require patterns, or masks, on the wafer, such that only certain areas of the wafer are affected. The desired patterns are produced on the wafer by a photolithography process which involves applying a photosensitive resistive material on the wafer, exposing with the desired pattern, and developing the wafer to remove the photosensitive resistive material.
During the photolithography process, alignment symbols are used to ensure that subsequent layers are correctly positioned over preceding layers. These alignment symbols, along with some test structures that permit general parametric testing, are normally placed in an overlap area between two adjacent sites. This overlap area is known as the scribe field and is created by overlapping a portion of the mask area with a previously exposed adjacent site each time the mask image is transferred to the wafer. Portions of the overlap area must not be doubly exposed when this occurs, or the alignment cues and test structures from the first exposure will be lost.
To prevent the loss of this information, one prior art solution is to create a single non-exposure window in the scribe area where the second exposure would normally occur. While this prevents the loss of information, it has certain drawbacks. For example, the die that are on the perimeter of the wafer have sections of their scribe in which no overlap occurs because there is not a neighboring exposure site. Thus, in a metal deposition process step, the area encompassed by the large non-exposure window, is covered with relatively large metal strips. After the wafer is cut into a plurality of die, or dice, the metal and the wafer substrate material have different coefficients of thermal expansion, which, when a die is heated, causes the metal strips to pull away from the substrate and bow upward. This bowing, when the die is attached to the package lead frame, can short contact structures of a package lead frame.
Another prior art solution to prevent the loss of the alignment symbols and test structures is to create many non-exposure windows as shown in FIG. 6. FIG. 6 illustrates a first scribe field, containing both alignment symbols and test structures, a second scribe field containing two non-exposure windows, and an overlap area. While this technique is an improvement over the single non-exposure window solution for perimeter dice, these windows can be large, thus exhibiting the same metal bowing problem described above.
In addition to the above mentioned difficulties, the non-exposure windows are manually generated based on the first scribe field. Thus, human error may cause the second scribe field to be incorrect, not generated, or not updated when the first scribe field changes. If any of these errors occur, the first scribe field information will be lost.
Therefore, a need exists for a method and apparatus that automatically generates a second scribe field and minimizes additional metallic material such that the metal bowing problem is eliminated.